In fiber-optic communication system development, there is a need to increase the capacity of a fiber transport connection. In the first optical communications systems virtually all transport was implemented by On-Off Keying (00K) of a single wavelength, and throughput was upgraded by increasing the symbol rate. This was followed by the introduction of parallelization into the fiber link by carrying many different data streams on separate wavelengths in the same optical fiber using wavelength-division multiplexing (WDM). Currently, wavelength counts in practical deployments have coalesced around 80-96 wavelengths per fiber at 50 GHz spacing, and have stopped increasing rapidly.
Other sophisticated modulation formats include Polarization-Multiplexed Quadrature Phase-shift Keying (PM-QPSK) and Orthogonal Frequency-Division Multiplexing (OFDM) that are used to simultaneously achieve data rates higher than the symbol rates and improved spectral efficiency.
FIG. 1 illustrates a conventional optical fiber 100. Optical fiber 100 is made up of concentric cylinders of glass and/or other materials. At the center of the optical fiber 100 is the core 102 which is a region of high refractive index where the electromagnetic field of the light is concentrated. Surrounding the core is the cladding 103, typically a region of lower refractive index than the core. The diameters and refractive indices of the core 102 and the cladding 103 are chosen so that the light is trapped by the core 102 and will not leak out of the fiber 100 as it propagates lengthwise along the fiber 100. The outermost layer, the coating 104, is applied to provide mechanical and chemical protection of the cladding from scratches and micro-bends, water penetration, etc. A typical single-mode fiber may have a 9 micron diameter core made of Germanium (Ge)-doped silica glass, a 125 micron diameter cladding made of undoped silica glass, and a 250 micron diameter coating of sophisticated polymer compounds. Because of the small core diameter, only a single transverse mode of the light is possible at standard wavelengths of 1310 nm to 1600 nm, for example, those typically used in telecommunications systems. The single-mode fiber can transmit two orthogonal polarizations, and each distinct wavelength may be considered as a separate longitudinal mode. The core 102, cladding 103, and coating 104 layer may be contained in an outer jacket 105, which provides additional mechanical strength and protection to the fiber 100.